This invention relates generally to a radiotelephone subscriber transceiver unit which may operate into more than one fixed system and more particularly to a mobile or portable radiotelephone transceiver unit having the capability of communicating messages between itself and a fixed site on a selected one of a number of radio channels. Since the transceiver may access more than one system sharing the same or adjacent radio channels, the present invention provides the apparatus and method for preventing unwanted operation in an undesired system.
One common multizone radiotelephone system is that known as a cellular radiotelephone system. A cellular radiotelephone system provides radiotelephone coverage over a relatively large geographic area by subdividing that area into small radio zones called cells. Each cell is provided a plurality of duplex radio channels (voice channels) over which radiotelephone messages may be carried. To control the operation of the subscriber units and to assign voice channels, each cell has a duplex radio channel dedicated to transmit and receive control information (called a aging channel). Further, to enable a subscriber to gain access to the system (and place a telephone call), a duplex radio channel is dedicated in each cell to allow a subscriber access to the system (called an access channel). Further definition of cellular radiotelephone systems may be found in EIA Interim Standard IS-3-D (March, 1987); "Cellular System, Mobile Station-Land Station Compatibility Specification", Electronic Industries Association.
In operation, a subscriber station first scans a stored list of control channels which are assigned one per cell and used by the cellular radiotelephone system to convey digital control information from the fixed equipment to subscriber stations and to convey digital control information from a subscriber station to the fixed equipment. (The control channel in a cell may also be used as a paging channel and as an access channel but in densely populated systems, the control channel is a separate radio channel). A multiple bit system identification (SID) is transmitted as part of the digital information and digital overhead from the fixed site on the control channel. The subscriber unit selects the control channel having the best signal strength, ceases scanning the list of control channels, and stores the SID in its memory. The subscriber unit then awaits an indication from the fixed site that it is being called or awaits an activation by the subscriber unit user to attempt a telephone call. The list of control channels is rescanned periodically (for example, every five minutes) or upon loss of signal or upon command from the fixed site.
Generally, cellular systems operated by different entities coordinate their control, paging, and access channel frequencies so that interference between independent systems in minimized. In fact, geographically adjacent systems utilizing the same band of radio frequencies may coordinate the independent system operation such that calls may be made in either system by a subscriber unit which is registered initially in only one system. Nevertheless, overlapping of the geographic radio coverage area of one cell of one system into the radio coverage area of another cell in the adjacent system can occur, thus enabling a subscriber unit to access the adjacent system when the user of the subscriber unit wishes only to access his "home" system. A subscriber unit attempting access into one system will leave a control channel to a certain access channel in the cell. For radio propagation or other reasons, the subscriber unit may receive an adjacent channel or co-channel radio frequency access channel from another system and be unable to complete its access.
A previous solution to a similar problem used the fact that two competing cellular systems for the same geographic area (system "A" offered by non-wireline service suppliers and system "B" offered by conventional landline telephone companies) use different radio frequency channels. The subscriber unit can be programmed to scan only those control channels available to either system A or system B. (See Motorola "Mini T*A*C 6800XL Cellular Mobile Telephone User's Manual", 68P81117E80-O, Aug. 14, 1987, pp. 40-44).
Further, as indicated on page 42 of the above User's Manual 68P81117E80-O, a user may enter specific SID numbers to limit operation to only certain cellular systems corresponding to the entered SIDs. Again, this solution prevents a subscriber unit from stopping on a control channel transmitting any but one of the entered SIDs. The problem of the subscriber unit inadvertently attempting access on an access channel which is not part of the desired cellular system remains unsolved.